U.S. Pat. No. 5,107,698 which issued to Gilliam in 1992 discloses a smoke generating apparatus. Smoke generating apparatus, such as those disclosed by Gilliam, are used to test for hairline cracks and similar leaks that are not detectable by visual inspection. The location of the leak is detected by observing smoke exiting from the leak. The Gilliam apparatus has a container in which is disposed a heating element. The container is filled with a smoke producing liquid until the liquid level reaches, without submerging, the heating element. An air pump forces a flow of air to bubble up through the smoke producing liquid, propelling a geyser of the smoke producing liquid into contact with the heating element. A portion of the smoke producing liquid coming in contact with the heating element is vaporized. When the smoke producing liquid is vaporized, it becomes smoke. As the flow of air exits the container it carries the smoke. The flow of air can be directed as required for testing purposes.
The bubbling up of air through the smoke producing liquid, as taught by Gilliam has a number of disadvantages. There have been recorded incidents of explosions occurring when the geyser of liquid served to complete a circuit through which electricity could flow.